This invention relates to a heat accumulator and more particularly to a heat accumulator using a latent heat accumulating material which holds a stable super-cooled condition at temperatures lower than the phase transformation temperature.
Heat accumulators, for storing heat energy and releasing it for use when required, fall into two categories: one type uses a sensible heat accumulating material, while the other type uses a latent heat accumulating material. A typical latent heat accumulating material discharges heat energy in accordance with a phase transformation thereof and has a heat accumulating density of about three to five times that of sensible heat accumulating material. As a result, heat accumulators using a latent heat accumulating material have an advantage in that they can be constructed in a compact form.
A well-known type of latent heat accumulating material is made of a substance which does not solidify and holds a stable supercooled condition at temperatures lower than the phase transformation temperature; for example, a sodium acetate-base hydrate. This material discharges its latent heat when it is released from supercooled condition and transformed from the liquid phase to the solid phase. Using this method, heat energy can be obtained from this heat accumulating material any time it is required.
To release this type of latent heat accumulating material from the supercooled condition, all that is normally necessary is to give an adequate stimulus to the latent heat accumulating material in supercooled condition. These supercooling release methods giving a stimulus include (1) touching the heat accumulating material with a sharp metal rod, (2) applying a voltage to the heat accumulating material via an electrode, and (3) adding a seed material to the heat accumulating material.
In the case of methods (1) and (2), however, the forming mechanism of a nucleus necessary for a release from the supercooled condition has not been fully clarified. In fact, with these methods, the heat accumulating material sometimes cannot be released from supercooled condition even if it is given a stimulus and therefore, they lack reliability.
On the other hand, method (3), which adds a nucleus necessary for the termination of supercooling, does ensure certain release from the supercooled condition. However, the seed material added to the heat accumulating material gradually accumulates therein, and eventually causes a separation of its phases, with the result that the heat accumulating material can no longer remain in a stable supercooled condition.
As described above, among the conventional supercooling releasing methods, those which give a mechanical or electrical stimulus lack reliability, while use of those which entail addition of a seed material result in degradation of the physical properties of the heat accumulating material.